Background Radiation Measurements at 400 meter Underground
for Dark Matter Search Study
T. Y. Kim, H. J. Kim, Y. C. Lee, E. Won, S. K. Kim
Department of Physics, Seoul National University, Seoul 151-742, Korea
Y. D. Kim, W. Y. So
Department of Physics, Sejong University, Seoul 143-747, Korea
Abstract
Recently we have performed measurements of background radiation, which is
crucial for any dark matter search experiments. We measured muons, neutrons, and
gamma backgrounds at approximately 400 meter underground tunnel in the electric
generating facility located about 120 km east of Seoul. We believe this may be the first
measurement at this depth in Korea. The muon flux measured with triple coincidence
between 3 scintillating plates was reduced by a factor of 104 compared with the flux at
ground level as expected at this depth. The unshielded gamma background measured
with 15 % relative efficiency germanium detector was rather high due to the
surrounding rocks. Shielded with 15 cm normal lead and 2.5 cm electrode copper gave
about 0.5 counts/second.
1. Introduction
Dark matter is believed to consist of more than 90 % of the mass of whole universe.
The non-baryonic candidates for the dark matter are the neutrino, axion, and weakly
interacting massive particle(WIMP)[1-2]. Currently there are much experimental efforts
for the dark matter search at extremely low background radiation. For WIMP search,
one utilizes the elastic scattering of WIMP with normal nuclei in the detector. People
have used Germanium, NaI(Tl), and low temperature thermal detectors. Searches for
WIMP can also be based on the detection of nuclear recoil energy in CsI(Tl) crystals. It
has been noted by several authors that CsI(Tl) crystal might have better performance for
recoil/ionizing particle separation[3]. Although the light yield is relatively lower than
NaI(Tl) crystal, the better separation can be more advantageous for WIMP search. Also
CsI(Tl) crystal has higher density, much less hygroscopicity than NaI(I). The spinindependent cross section of WIMP is larger for CsI(Tl) than NaI(Tl) because CsI(Tl)
has a compound with 2 heavy nuclei while spin-dependent cross section will be
comparable. Moreover hundreds of tons of CsI(Tl) crystals are already being used for
several detectors in high energy experiment[4]. Thus fabricating large amount of
crystals will be quite feasible. We already demonstrated [5] that low energy gamma rays
down to few keV was detected with CsI(Tl) crystal detector. A clear peak at 6 keV was
observed using X-ray source. Good energy resolution and linearity had been achieved
up to X-ray region. We also showed that alpha particles and gamma rays could be
clearly separated using the different time characteristics of the crystal.
On the other hand, the interaction of the WIMP-nucleus elastic scattering in the
target material is, in principle, a rare process. If the environmental radiation background
or electric noise are much larger than the nuclear recoil signal, the WIMP signal will be
overwhelmed by them even though the detector is fully sensitive to low energy nuclear
recoil[6]. Thus extremely low background levels are essential for this experiment. The
following lists are the major sources of the background: 1) environmental radioactivity,
2) radioimpurities in detector and shielding material, and 3) cosmic rays. The gamma
radiation from the environment is comprised almost exclusively of photons from the
primordial U and Th decay series and from 40K decay. In order to reduce the gamma
radiation, we have to consider how thick and what kind of material should be used in
shielding. Interactions between the primary cosmic rays and the atmospheric atoms
generate many kinds of the elementary particles such as neutrons, electrons, neutrinos,
protons, muons, pions, and so on. Hadrons and photons are directly absorbed by the
shield or by overburden from build ceilings. Owing to the small neutrino-hadron cross
section, a detector having small target volume can not see their interactions. Only
muons and neutrons produced in shield by muons are relevant particles in low
background techniques. Considering economy of time and labor, deep underground is
the best place for reducing the background of cosmic rays in the low background
experiment.
This work is the test experiment in order to study the requirements of an
underground facility for the WIMP search.
2. Test setup
BC408
PMT1
PMT2
Dim. = 17x50x1.5 cm3
Cosmic-ray
PMT3
Start
COIN
Gate
PMT1
ADC1
TDC1
PMT2
ADC2
TDC2
PMT3
Fiq. 1.
Schematic of the muon detection system.
TDC3
We measured muons, neutrons, and gamma backgrounds at the 400 meter underground
tunnel in the underground pumped storage power plant (Yangsu Baljeoncho,
Chongpyong, Korea) located about 120 km east of Seoul. We believe this may be the
first measurement at this depth in Korea. The instruments used in the test are 3 plastic
scintillator counters for muon, a NE213 liquid scintillator counter for neutron, and a
HPGe detector of 15 % relative efficiency as a gamma ray spectrometer.
Schematic of the muon counter is shown in Fig. 1. The coincidence of PMT1 and PMT2
signals gave the TDC start time and generated a gate for the ADC. Fig. 2 shows the
neutron detection system using NE213. Photomultiplier tube (PMT) of 2 inch diameter
(Hamamatsu H1161) was attached directly on the cylindrical cell filled with liquid
scintillator NE213. The surface attached to the PMT was entirely covered by the
photocathode window. And the conventional electronics for the gamma ray spectrometer,
shown in Fig. 3, was applied to the gamma ray background measurement.
TDC
Start
Constant
Fraction
Disc.
NE213
Gate
Generator
Delay
Line
Amp.
Pulse
Shape
Analyzer
ADC
Gate
Gate
Generator
TDC
Stop
ADC
Fig. 2. A Neutron detection system with pulse shape discrimination.
ADC
Amp
HPGe
Detector
Timing
Amp
DISC
(10mV)
Gate
Generator
(5 s)
Veto
(0.2 ms)
Fig. 3. A system for gamma ray spectroscopy.
ADC
Gate
A CAMAC system is used for the data taking with CAMAC interface to a PC
whose operating system is LINUX. We developed DAQ system with CAMAC interface
based on the ROOT package for this experiment and entire analysis was performed with
the ROOT package too[7].
3. Test results
3.1 Muons
Á¦¸ñ:
c 1.eps
ÀÛ¼º ÇÑ »ç ¶÷:
ROOT Version 2.22/10
¹Ì¸® º ¸±â:
ÀÌ EPS ±×¸²Àº ¹Ì¸® º ¸±â ±×¸²À» Æ÷ÇÔÇÏÁö ¾ Ê°í
ÀúÀåµÇ¾ú½À´Ï´Ù.
¼³¸í:
ÀÌ EPS ±×¸²Àº Pos tSc ript ÇÁ¸°Å͸¦
Á¦¿ÜÇÑ ´Ù¸¥ ÇÁ¸°ÅÍ¿¡¼-´Â
ÀμâµÇÁö ¾ ʽ À´Ï´Ù.
Fig. 4. Distribution of the deposited energy in the scintillation detector with a) double
coincidence and b) triple coincidence at ground level.
In general, energy loss by ionization for high energy particle is constant. In case of
muon, this value is about 2.5 MeV g-1cm-2. Fig. 4 shows the distribution of the total
deposited energy in the scintillation detector 1 and 2, in Fig. 1, with a) double
coincidence and b) triple coincidence at ground laboratory. The contamination of the
low energy Compton scattered electrons from the environmental gamma rays was
inevitable one in our geometry of detection system. But the low energy electrons below
2 MeV can not pass through 3 plastic scintillation plates and the probability of the
sequential Compton scattering with every scintillation plate is very small. Events
triggered with triple coincidence show the definite asymmetric Landau distribution with
a high energy tale.
Using the coordinates of our detection system shown in Fig. 5., the solid angle
subtended by two rectangles of our detection system was calculated by the numerical
calculation of the following integral[8].
a
b
( w x p )( h  y p )
a
b
z p [( w x p ) 2  ( w y p ) 2  R 2 ]1 / 2
   dx  dy{arctan
 arctan
( w x p )( h  y p )
z p [( w x p ) 2  ( w y p ) 2  R 2 ]1 / 2
 arctan
 arctan
( w x p )( h  y p )
z p [( w x p ) 2  ( w y p ) 2  R 2 ]1 / 2
( w x p )( h  y p )
z p [( w x p ) 2  ( w y p ) 2  R 2 ]1 / 2
}
The measured cosmic ray fluxes were 0.51 cm-2sr-1s-1 at ground level and 0.25 x 10-4
cm-2sr-1s-1 at 400 m underground. These are comparable to the values referred in Ref.
[9], where the vertical muon flux at ground level is 0.71 cm-2sr-1s-1.
-b
-a
P(xp,yp,R)
a
b
R
-h
-w
Y
w
X
h
Z
Fig. 5. Geometry for the solid angle subtended by two rectangular scintillators.
3.2 Neutrons
The apparatus was a cylindrical cell filled with liquid scintillator NE213. The light
emission of the liquid scintillator NE213 exhibit a substantial slow component, which
depend on the specific energy loss of the particle, as well as a signal of fast decay
component. Neutron-initiated recoil proton signals were effectively separated from
electron signals originating from gamma rays with the use of pulse shape discrimination
(PSD). A neutron source of 252Cf was used for PSD calibration. 3.1 % of 252Cf decay is
spontaneous fission. About 20 ’s (80 % < 1 MeV) and 4 neutrons ( <En> = 2.14 MeV)
are produced every fission. A calibration spectrum of the ADC versus the TDC is shown
in Fig. 6 a). This plot illustrated the good separation between neutron and gamma ray
signal. Fig. 6 b) and Fig. 6 c) show the spectrum of the ground measurement and the
underground measurement without shield. Their count rates were 0.0050 sec-1 at ground
level and 0.0015 sec-1 at 400 m underground, respectively.
Á¦¸ñ:
neutron_f.eps
ÀÛ¼º ÇÑ »ç ¶÷:
ROOT Version 2.22/10
¹Ì¸® º ¸±â:
ÀÌ EPS ±×¸²Àº ¹Ì¸® º ¸±â ±×¸²À» Æ÷ÇÔÇÏÁö ¾ Ê°í
ÀúÀåµÇ¾ú½À´Ï´Ù.
¼³¸í:
ÀÌ EPS ±×¸²Àº Pos tSc ript ÇÁ¸°Å͸¦
Á¦¿ÜÇÑ ´Ù¸¥ ÇÁ¸°ÅÍ¿¡¼-´Â
ÀμâµÇÁö ¾ ʽ À´Ï´Ù.
Fig. 6. Two-dimensional spectra of the ADC versus the TDC. A) a spectrum for PSD
calibration using 252Cf source, B) a spectrum measured at ground, and C) a spectrum measured
at 400 m underground.
3.3 Gamma rays
We performed several measurements with different shieldings. In Fig. 7., spectra of
HPGe detector at ground level and at 400 m underground are compared. More than 40
peaks were found in the spectrum of the unshielded underground measurement and the
rates of individual peaks of primordial radionuclides are given in table 1. The integral
count rates (60 - 1600 keV) of each spectrum corresponded to a) 310 Hz for unshielded
underground, b) 112 Hz for unshielded ground measurements, and c) 5.05 Hz for 5cm
lead, and d) 0.53 Hz for 15cm together with 2.5cm copper shielded measurement,
respectively. The general appearance of the background spectra showed all activities
from the environmental radionuclides of underground were increased rather than ground.
And shielding of 15 cm Pb together with 2.5 cm electric copper inner shield reduced the
background radiation level by a further factor of 500. In both cases of spectra measured
with shield, peaks having strong intensity are from progenies of 222Rn, such as 214Bi and
Á¦¸ñ:
c 1.eps
ÀÛ¼º ÇÑ »ç ¶÷:
ROOT Version 2.21/01
¹Ì¸® º ¸±â:
ÀÌ EPS ±×¸²Àº ¹Ì¸® º ¸±â ±×¸²À» Æ÷ÇÔÇÏÁö ¾ Ê°í
ÀúÀåµÇ¾ú½À´Ï´Ù.
¼³¸í:
ÀÌ EPS ±×¸²Àº Pos tSc ript ÇÁ¸°Å͸¦
Á¦¿ÜÇÑ ´Ù¸¥ ÇÁ¸°ÅÍ¿¡¼-´Â
ÀμâµÇÁö ¾ ʽ À´Ï´Ù.
Fig. 7. Background spectra measured a) without shield, c) 5cm Pb, d) 15cm Pb + 2.5 cm Cu
shield at 400 meter underground, and b) without shield at ground.
Table 1. Count rates of the peaks measured without shield at 400 meter underground.
Energy
(keV)
Count rate
( 10-3 s-1 )
Energy
(keV)
Count rate
( 10-3s-1 )
72.8/75
85/87
92.5
185.7/186.2
209.3
238.6
270.2
277.4
295.2
327.6
338.4
351.9
409.5
438.8
463.0
510.8/511
582.2
609.3
665.5
727.2
755.2
768.4
400.7
1331
72.7
77.5
92.7
1257.4
113.9
39.4
375.6
74.9
255.8
591.5
32.6
24.1
91.5
238.6
527.7
624.7
29.2
112.9
15.4
39.2
785.9
794.7
806.2
860.4
911.1
934.1
949.8
969.1
1120.3
1155.2
1238.1
1281.0
1377.7
1385.3
1401.5
1408.0
1460.8
1495.8
1509.3
1592.7
23.8
65.5
17.2
67.8
367.7
35.8
10.3
149.5
156.1
18.02
58.8
9.35
38.4
9.35
5.12
18.7
1747.5
7.12
20.3
100.1
Table 2.
Count rates of the main peaks of the measured spectra.
Energy (keV)
185.7/186.2
238.6
351.9
582.2
609.3
661.7
911.1
1120.2
1460.8
Nuclide
235
U/226Ra
Pb(Th)
214
Pb(U)
208
Tl(Th)
214
Bi(U)
137
Cs
228
Ac(Th)
214
Bi(U)
40
K
212
Underground( 10-3s-1 )
15cm Pb +
5cm Pb
Unshield
2.5 cm Cu
5.8  0.2
4.7  0.3
77.5  2.1
2.1  0.1
2.6  0.2
1257.4  5.3
2.4  0.2
1.9  0.1
591.5  5.2
0.38  0.07 3.0  0.1
527.7  4.9
1.9  0.1
2.9  0.1
624.7  5.3
8.7  0.3
13.1  0.4
367.7  4.1
0.56  0.07 4.1  0.1
156.1  2.6
0.47  0.07 57.0  0.9
1747.5  8.9
Ground
(unshield)
( 10-3s-1 )
80.8  1.1
541.6  3.6
289.16  2.4
247.9  2.1
267.6  2.1
183.5  1.7
52.5  0.9
1171.5  4.5
214
Pb. This background was coming from air and couldn’t reduce by lead. We believe
this background can be eliminated by N2 gas flow. The count rates of main peak
characterizing each background spectrum are tabulated in Table 2. Surprisingly, we
observed the fact that the 662 keV gamma ray peak of 137Cs could not be seen in the
spectrum measured without shield, but clearly seen in the spectrum measured with lead
shield. We believed that lead used in this shielding had contamination of 137Cs.
4. Discussions and conclusion
After a test of CsI(Tl) crystal for the dark matter search, we performed another test
to study the dominant background sources at 400 m underground.
Preliminary test results were given in section 3. Firstly, the measured cosmic ray
fluxes were 0.51 cm-2sr-1s-1 at ground level and 0.25 x 10-4 cm-2sr-1s-1 at 400 m
underground. Muon flux was reduced to be 10-4 level when we measured it underground.
Secondly, we measured neutron background with the cylindrical scintillation cell whose
diameter was 2 inch. The test result was not reliable because of low statistic. For more
precise measurement of muon-induced neutrons in shielding material and neutrons from
(,n) reactions as well as external neutrons, A detector that has large effective volume to
the neutron is needed. We have a plan to manufacture a 4 liter cubic scintillation cell
filled with NE213 viewed by multiple PMTs. Finally, spectra of various shielding
conditions were obtained with the HPGe detector. From this, we need more pure
radioactive shielding material and technique to reduce Rn and its progenies.
Acknowledgements
We wish to thank the Underground Pumped Storage Power Plant at Chongpyong
and its employees for their warm welcome and indispensable assistance at the
underground power plant. we are also grateful to Prof. J.C. Kim for supporting this
work and the loan of the HPGe detector.
References
[1] G. Jungman et al., “Supersymmetric Dark Matter”, Phys. Reports 267 (1996)
[2] G.G. Raffelt, hep-ph/9712538, Dec 1997.
[3] J.B. Birks, Theory and practice of scintillation counter, (Pergamnon press, Oxford,
1964).
[4] E. Blucher et al. (CLEO), Nucl. Intrum. Methods A235, 319 (1985); M.T.Cheng
et al. (BELLE), Technical Design Report, KEK Report 95-1, 1995.
[5] H.J.Kim et al., submitted to the Proceeding of International Conference on High
Energy Physics, Vancouver (1998); E.Won et al., submitted to the Nuclear
Physics B(Proceedings Supplements) (1998)
[6] G. Heusser, Annu. Rev. Nucl. Parti. Sci. 45, 543 (1995)
[7] R. Brun and F. Rademakers, ROOT - An Object Oriented Data Analysis Framework,
Proceedings AIHENP'96 Workshop, Lausanne, Sep. 1996; Nucl. Intrum. Methods
A389 (1997)
[8] J. Cook, Nucl. Intrum. Methods A178, 561 (1997)
[9] Particle Data Group, “the Review of Particle Physcis”, edition of 1998.